In recent years, a technique for forming a thin film transistor (also referred to as a TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
A thin film transistor is manufactured mainly by using a semiconductor material such as amorphous silicon or polycrystalline silicon. TFTs using amorphous silicon have low field effect mobility but can be formed over a large glass substrate. On the other hand, TFTs using polycrystalline silicon have high field effect mobility, but require a crystallization step such as laser annealing and are not always suitable for being formed over a large glass substrate.
Thus, a technique in which a TFT is formed using an oxide semiconductor as a semiconductor material and applied to an electronic device or an optical device has attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used as a switching element or the like in an image display device.
A TFT in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have higher field effect mobility than a TFT using amorphous silicon. An oxide semiconductor film can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and a manufacturing process thereof is simpler than that of a TFT using polycrystalline silicon.
TFTs which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.
Moreover, there is a trend in an active matrix semiconductor device typified by a liquid crystal display device towards a larger screen, e.g., a 60-inch diagonal screen, and further, the development of an active matrix semiconductor device is aimed even at a screen size of a diagonal of 120 inches or more. In addition, a trend in resolution of a screen is toward higher definition, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and prompt development of a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2180, is also pushed.
As a display device has a higher definition, the number of pixels needed for it is significantly increased. As a result, writing time for one pixel is shortened, and thus a thin film transistor is required to have high speed operation characteristics, large on current, and the like. In the meantime, a problem of energy depletion in recent years has caused demand for a display device whose power consumption is suppressed. Therefore, a thin film transistor is also required to have low off current and suppressed unnecessary leakage current.
Increase in screen size or definition tends to increase wiring resistance in a display portion. Increase in wiring resistance causes delay of signal transmission to an end portion of a signal line, drop in voltage of a power supply line, or the like. As a result, deterioration of display quality, such as display unevenness or a defect in grayscale, or increase in power consumption is caused.
In order to suppress increase in wiring resistance, a technique of forming a low-resistance wiring layer with the use of copper (Cu) is considered (e.g., see Patent Documents 3 and 4).